A conventional vehicle air-conditioning system includes a variable capacity compressor to control the temperature at the evaporator. This system is operative to calculate the deviation of an actual evaporator blowing temperature Te sensed by a temperature sensor from a target evaporator temperature TEO. Next, a capacity control current for the compressor is calculated based upon the above deviation. The capacity control current is delivered to a capacity varying device of the compressor to thereby variably controlling the discharge capacity of the compressor. This allows the compressor to increase or decrease its discharge flow rate of refrigerant and control the cycle low pressure (evaporation pressure of refrigerant), thereby maintaining the evaporator blowing temperature Te at or near the target evaporator temperature TEO.
The aforementioned system relates to a single air conditioner that only includes an air-conditioning unit on the front seat side in the passenger compartment as a unit for air-conditioning the passenger compartment. Since this single air conditioner is provided with only one evaporator in the refrigeration cycle, the aforementioned capacity control raises no significant problem.
However, performance and results are different when the refrigeration cycle is of a dual air conditioner type in which an air-conditioning unit is disposed both on the front and rear seat sides in the passenger compartment as units for air-conditioning the passenger compartment. In this case, since the evaporator of the front seat air-conditioning unit and the evaporator of the rear seat air-conditioning unit are arranged in parallel with a compressor and commonly use a condenser, the refrigerant discharged from the one variable capacity compressor is circulated in parallel through the front and rear seat evaporators.
In the refrigeration cycle of the dual air conditioner type, the front seat air-conditioning unit is usually regarded as the main air-conditioning unit. In this context, the inventors of the present invention first built and studied a prototype of a system that provides capacity control to the variable capacity compressor in accordance with the deviation between the actual evaporator blowing temperature Tef and the target evaporator temperature TEO at the front seat evaporator.
As a result, it was found that a worsened distribution balance between the refrigerant flow rate for the front seat evaporator and that for the rear seat evaporator led to a lack in refrigerant flow rate of the rear seat evaporator, thereby causing a significant increase in the temperature of air blown into the rear seat area when compared with the front seat area. This makes a passenger in the rear air-conditioned seat feel less comfortable.
More specifically, an intermediate season such as the spring or fall season requires less cooling heat load for the vehicle air-conditioning system due to lower temperatures of outside air than in the summer season as well as less need for defogging of the windshield of the motor vehicle than in the winter season. In this context, the air-conditioning system employs power saving control (economy control) in which during the intermediate seasons, the target evaporator temperature TEO is set at a temperature higher than during the summer (e.g., by about 11° C.) to reduce the discharge capacity of the compressor thereby reducing the power for driving the compressor.
During this power saving control operation, the discharge capacity of the compressor is reduced thereby causing a decrease in the flow rate of a refrigerant circulating within the cycle. In this case, like a recreational vehicle of minivan type, a vehicle having a rear seat area larger than a front seat area in the passenger compartment space has generally a higher cooling heat load on the rear seat side than on the front seat side. However, from the viewpoint of cooling capability, the front seat evaporator, which is always used, is designed to have a larger capability than that of the rear seat evaporator. Accordingly, the front seat evaporator is greater in capacity than the rear seat evaporator, and the pressure loss in a front seat expansion valve is less than the pressure loss in the capacity of the rear seat evaporator.
During the power saving control operation, this may result in an extremely reduced flow rate of the refrigerant circulating through the rear seat evaporator, thereby causing the temperature of air blown into the rear seat area to be significantly increased, e.g., by about 8° C. when compared with the front seat area and thus leading to a lack in cooling of the rear seat area.
On the other hand, the inventors also studied a vehicle air-conditioning system of the dual air conditioner type which employs as its compressor not a variable capacity compressor but a motor driven compressor that is capable of using RPM control for the control of its discharge flow rate of refrigerant. The study showed that this system also had the problem of the temperature of air blown into the rear seat being significantly increased relative to the temperature of air blown into the front seat at a low flow rate of refrigerant.